Read only memory (ROM) devices are semiconductor integrated circuits widely used in microprocessor-based systems to permanently store information even when power is off. ROM devices are particularly well suited for applications where a large volume of devices having identical data are required or for storing data that is repeatedly used. An example of such an application is the BIOS on personal computers. ROM devices store binary signals as an array of active elements that are typically programmed as part of the fabrication process by the integrated circuit manufacturer according to a customer's specifications.
Conventional mask ROM devices include NOR-type and NAND-type. A NOR-type ROM is formed by connecting in parallel the sources and the drains of the memory transistors. Alternatively, connecting the sources and the drains of the memory transistors in series forms a NAND-type ROM.
As shown in FIGS. 1-3 fabrication of a conventional flat-cell mask ROM begins with a semiconductor silicon substrate (10) doped with P-type impurities. Buried bit lines (11) that will constitute source/drain regions are formed by implanting N-type impurities into multiple parallel strip shaped regions of the substrate. A gate oxide layer (12), typically silicon oxide formed by thermal oxidation, is then formed over the substrate (10). Gate electrodes (13) are then formed orthogonal to the buried bit lines (11), constituting word lines for the memory array of the mask ROM device. Convention coding procedure requires that a photoresist layer (14) be applied covering the surface of the substrate (10) while leaving the coding openings (15) exposed. Impurity ions are then implanted into the exposed channel regions of the selected memory cells.
The channel regions for the memory cell transistors lies in the region of the substrate between every two adjacent bit lines beneath the word lines. The memory cell transistors are coded as either blocking or conducting. A 1 or 0 data bit can be defined as either state. If a cell is implanted with P-type impurities, the cell is set to have a high threshold voltage effectively setting the memory cell to a permanently OFF state representing, for example, the storage of binary digit of 0. Cells without implanted impurities have a low threshold voltage setting the memory cell to a permanently ON state representing, for example, the storage of a binary 1.
As a result of semiconductor device manufacturers striving to improve performance and reduce cost, the size of ROM devices continues to shrink while the density of ROM devices continues to increase. A problem arises because the reduced space between adjacent bit lines makes leakage current arising from the P-type implants and word line parasitic capacitance relatively more detrimental.
U.S. Pat. No. 5,504,036 to Chung et al. discloses one solution to these problems. By using a liquid-phase deposition process to form a thick code oxide, Chung et al. produce permanently OFF cells on the semiconductor substrate. A liquid-phase deposition process, however, cannot easily be integrated into CMOS manufacturing.
U.S. Pat. No. 5,480,823 to Hsu discloses the use of a thick oxide to produce normally OFF cells. Hsu teaches the growth of a thick oxide layer on areas of a substrate that have been defined by photolithography and subsequent etch of a silicon nitride layer. Growth of the thick oxide in this manner, however, produces a "birds beak" on the buried bit line edge.
In light of the foregoing, there is a need for a process to manufacture ROM with a thick code oxide that can be easily integrated into standard CMOS manufacturing.